Head structure for thermally-assisted recording (TAR) disk drive

ABSTRACT

A thermally-assisted recording (TAR) head structure has a semiconductor substrate as the head carrier with a vertical-cavity surface-emitting laser (VCSEL) on its front surface, a TAR head formed directly on the VCSEL, and a highly reflecting third mirror on its back surface. The semiconductor substrate serves as an extended cavity for the VCSEL. The TAR head is fabricated on the outer surface of the VCSEL in the same manner as proposed for fabrication of a TAR head on a conventional slider. The TAR head includes a conventional read head and write head, and an optical waveguide with a grating coupler and a near-field transducer (NFT). The laser radiation is output through a partially reflecting output mirror of the VCSEL through the front surface to the grating coupler, which turns the incoming laser radiation 90 degrees and directs it into the waveguide from where it is directed to the NFT.

TECHNICAL FIELD

This invention relates generally to a thermally-assisted recording (TAR)disk drive, in which data are written while the magnetic recording layeron the disk is at an elevated temperature, and more specifically to aTAR head structure that includes a laser, an optical waveguide, themagnetic write head and the magnetoresistive read head integrated withthe head carrier.

BACKGROUND OF THE INVENTION

Thermally-assisted recording (TAR), also called heat-assisted magneticrecording (HAMR), has been proposed. In a TAR disk drive, an opticalwaveguide with a near-field transducer (NFT) directs radiation from alaser to heat localized regions of the magnetic recording layer on thedisk. The radiation heats the magnetic material locally to near or aboveits Curie temperature to lower the coercivity enough for writing tooccur by the magnetic field from the write head. The recorded data isread back by a conventional magnetoresistive read head. The opticalwaveguide, write head and read head are formed on the trailing surfaceof a head carrier, such as a slider with an air-bearing surface (ABS)that allows the slider to ride on a thin film of air above the surfaceof the rotating disk.

One of the challenges for TAR is the integration of the laser with thehead carrier so that the laser light is directed to the opticalwaveguide. A vertical-cavity surface-emitting laser (VCSEL) has beenproposed because of its high reliability and low cost. TAR sliders withvarious means for attachment of a VCSEL are described in US 20080002298A1 and US 20090310459 A1. However, a typical VCSEL does not haveadequate power output for currently proposed TAR disk drives.

An extended cavity VCSEL has been proposed where a third mirror is onthe back side of the VCSEL semiconductor substrate. The thickness of thesemiconductor substrate forms an extended cavity for the VCSEL, whichallows for higher single mode power than can be achieved with a typicalVCSEL without the external cavity and third mirror. An extended cavityVCSEL is described in U.S. Pat. No. 6,778,582 B1.

What is needed is a TAR head structure with an extended cavity VCSELintegrated with the head carrier.

SUMMARY OF THE INVENTION

The invention relates to a TAR head structure wherein the head carriercomprises an extended cavity VCSEL and the TAR head is fabricated on theouter surface of the VCSEL. The head carrier is a semiconductorsubstrate with front and back surfaces and a surface substantiallyorthogonal to the front and back surfaces that function as the ABS. AVCSEL is formed on the semiconductor substrate front surface, and ahighly reflecting third mirror is formed on the semiconductor substrateback surface so that semiconductor substrate serves as the extendedcavity. The laser radiation is output through a partially reflectingoutput mirror through the front surface. The TAR head is fabricated onthe outer surface of the VCSEL in the same manner as proposed forfabrication of a TAR head on a conventional slider. The TAR head has anABS that is substantially coplanar with the ABS of the semiconductorsubstrate. The TAR head includes a conventional magnetoresistive readhead, a write head with write pole and electrically conductive coil, anoptical waveguide with a grating coupler and a NFT located at the ABS,and conventional insulating layers, typically alumina, surrounding andelectrically isolating the various TAR head elements. During fabricationof the TAR head directly on the VCSEL, vias are formed in the insulatinglayers to bring electrical connection to the VCSEL from pads on an outersurface of the TAR head, in the same manner that electrical connectionis made to the read head and coil of the write head. The laser radiationfrom the extended cavity VCSEL is output through theradiation-transmissive insulating layers of the TAR head to the gratingcoupler, which turns the incoming laser radiation 90 degrees and directsit into the waveguide from where it is directed to the NFT at the ABS.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is sectional view of a prior art extended cavity vertical cavitysurface emitting laser (VCSEL).

FIG. 2 is a sectional view through a portion of a disk and anair-bearing slider that supports a thermally-assisted recording (TAR)head according to the prior art.

FIG. 3 is a sectional view of the TAR head structure according to theinvention and shows a sectional view of a portion of a perpendicularmagnetic recording disk maintained near the disk-facing surface of thehead structure.

FIG. 4 is a view of the grating coupler and waveguide portion of the TARhead structure according to the invention as viewed from a directionorthogonal to the outer surface of the VCSEL.

FIG. 5 is a perspective view of a portion of a semiconductor wafer withgenerally rectangular regions, each region having a patterned VCSEL andTAR head, prior to cutting of the wafer into the individual TAR headstructures according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a sectional view of a monolithic extended cavity VCSELaccording to the prior art and as described in U.S. Pat. No. 6,778,582B1. The extended cavity VCSEL has a semiconductor substrate 10 withfront surface 10 a, back surface 10 b and thickness L. A VCSEL withactive region 1 is formed on front surface 10 a and includes gain layer16 located between partially reflecting intermediate Bragg reflector ormirror 14 and bottom Bragg mirror 18, and an oxide layer 22 that definesan aperture 22 a. A partially reflecting output mirror 28 is formed onthe back surface 10 b of semiconductor substrate 10 and functions as athird mirror. The laser radiation is emitted through the third mirror28. The semiconductor substrate 10 with thickness L functions as anextended cavity for the VCSEL. The extended cavity allows for highersingle mode power than can be achieved with a typical VCSEL without theextended cavity and third mirror. When the extended cavity is made fromGaAs, the extended cavity VCSEL may be designed to generate laserradiation with a wavelength of greater than approximately 920 nm. Forexample, the wavelength may be between 920 nm and 1000 nm. Shorterwavelengths require the use of a different substrate due to opticallosses in the in GaAs. The extended cavity VCSEL shown in FIG. 1 is asingle device that has been cut from a semiconductor wafer onto whichthe materials making up the various layers have been deposited andpatterned using well-known semiconductor lithographic and fabricationprocesses. Thus a large number of devices are formed on a single wafer.The materials, dimensions and fabrication methods for the extendedcavity VCSEL shown in FIG. 1 are described in detail in U.S. Pat. No.6,778,582 B1.

FIG. 2 is a sectional view through a portion of a TAR disk 100 andair-bearing slider 110 that functions as the head carrier to support theTAR head, as proposed in the prior art. FIG. 2 is not drawn to scalebecause of the difficulty in showing the very small features. The TARdisk 100 is depicted as a patterned-media disk with a disk substrate 118and discrete magnetic islands 130 and nonmagnetic regions 132. Theislands 130 are spaced apart by nonmagnetic regions 132, which mayformed of polymeric material for planarizing disk 100. The islands 130are magnetized perpendicularly, resulting in the recorded bits beingstored in the recording layer of the islands 130 in a generallyperpendicular or out-of-plane orientation. The islands 130 are discretemagnetic islands that function as the patterned bits. A heat sink layer121 may located below the islands 130 and nonmagnetic regions 132. TheTAR disk 100 may also be a conventional continuous-media magneticrecording disk wherein the recording layer is not patterned but is acontinuous layer.

Also shown on slider 110 with disk-facing surface or air-bearing surface(ABS) is the read head 60 and the write head 50 (with the yoke thatconnects write pole 52 and a return pole 54). The ABS of slider 110 isthe surface that faces the disk 100 and is shown without the thinprotective overcoat typically present in an actual slider. The ABS shallmean the surface of the head carrier that is covered with a thinprotective overcoat, the actual outer surface of the head carrier ifthere is no overcoat, or the outer surface of the overcoat. Writecurrent passes through a coil 56 of the write head 50 to generate amagnetic field (arrow 42) at the write pole 52. This magnetic fieldmagnetizes the recording layer on the island 130 beneath the write pole52 as the disk 100 moves past the write head 50 in the direction ofarrow 123. The detection or reading of the recorded bits is by a readhead 60 having a sensing edge 60 a at the ABS. The read head istypically a magnetoresistive (MR) read head, such as a tunneling MR(TMR) read head in which a sense current passes perpendicularly throughthe layers making up the head. A pair of magnetic permeable shields S1and S2 are located on opposite sides of read head 60 to prevent magneticflux from bits other than the bit being read from reaching the read head60. The write coil 56 is shown as a conventional flat or “pancake” coilwrapped around the yoke that connects the write pole 52 with the returnpole 54, with the electrical current directions being shown as into thepaper by the coil cross-sections marked with an “X” and out of the paperby the coil cross-sections marked with a solid circle. However, the coilmay also be a conventional helical coil wrapped around the portion ofthe yoke that directly supports the write pole 52. The TAR head has anouter surface 115 with electrically conductive pads (not shown) thatconnect through the insulating layers 113 to the read head 60 and coil56 of write head 50.

The slider 110 also supports a laser 70, mirror 71, optical channel orwaveguide 72 and near-field transducer (NFT) 74, which has its output atthe ABS. The optical waveguide 72 is depicted in FIG. 2 as extendingthrough the yoke of write head 50 and being located between the writepole 52 and return pole 54. However the optical waveguide 72 may belocated at other locations, such as between shield S2 and return pole54, or between the write pole 52 and the outer face of the slider 110.The waveguide 72 is formed of a core material such as Ta₂O₅ or anotherhigh index dielectric material that is transmissive to radiation at thewavelength of the laser and is surrounded by a dielectric cladding layer73 (for example SiO₂ or Al₂O₃) of lower refractive index than the corematerial. While the slider 110 in FIG. 2 is depicted as supportingmirror 71 for directing the laser radiation from laser 70 into waveguide72, it is known to use a grating coupler coupled to the waveguide, asdescribed for example in US 20090310459 A1.

The NFT 74 is located at the output of waveguide 72 at the ABS of theslider 110. The laser radiation strikes the NFT 74, creatingconcentrated near-field radiation to the islands 130 as the disk rotatesin the direction 123 past the slider 110. A “near-field” transducer, asused herein, refers to “near-field optics”, wherein the passage of lightis to, from, through, or near an element with subwavelength features andthe light is coupled to a second element located a subwavelengthdistance from the first. NFTs typically use a low-loss metal (e.g., Au,Ag, Al or Cu) shaped in such a way to concentrate surface charge motionat a surface feature shaped as a primary apex or tip. Oscillating tipcharge creates an intense near-field pattern. Sometimes, the metalstructure can create resonant charge motion, called surface plasmons orlocal plasmons, to further increase intensity. The electromagnetic fieldof the oscillating tip charge then gives rise to optical output in thenear field, which is directed to the data islands on the disk. The NFT74 has features less than the wavelength of the laser radiation and thespacing between the NFT 74 and the islands is less than the wavelengthof the laser radiation.

When write-current is directed through coil 56, the write pole 52directs magnetic flux to the data islands 130. The dashed line 117 witharrows shows the flux return path back to the return pole 54. The NFT 74directs near-field radiation, as represented by wavy arrow 82, to thedata islands 130 as the TAR disk 100 moves in the direction 123 relativeto the slider 110. The electric charge oscillations in the NFT 74 heatthe data islands 130. This raises the temperature of the magneticrecording material in a data island to near or above its Curietemperature to thereby lower the coercivity of the material and enablethe magnetization of the data island to be switched by the write fieldfrom the write pole 52.

The read head 60, shields S1, S2, return pole 54, write pole 52, coil 56and waveguide 72 are fabricated on the trailing surface 112 of slider110 using well-known conventional thin film deposition and patterningtechniques. Insulating material, typically alumina, is deposited atvarious times during the fabrication process to separate the various TARhead elements and refill recessed areas, as shown by insulating layers113. The insulating material generally surrounds the TAR head elementsand provides a portion of the ABS. The slider 110 is typically formed ofan alumina/titanium-carbide (Al₂O₃/TiC) composite material. The trailingsurface 112 is the surface of a wafer onto which a large number of TARheads are patterned. The wafer is then diced into individual sliderswith the length of the sliders (in the direction perpendicular totrailing surface 112) corresponding generally to the thickness of thewafer. US 20090258186 A1, assigned to the same assignee as thisapplication, describes a wafer-level process for fabricating TAR headswith thin film waveguides and NFTs.

The invention relates to a TAR head structure wherein the head carriercomprises an extended cavity VCSEL and the TAR head is fabricated on theouter surface of the VCSEL. An embodiment of the invention is shown inthe sectional view of FIG. 3, which is not to scale. The head carrier issemiconductor substrate 210 with front surface 210 a and back surface210 b and thickness L1. The substrate has an ABS substantiallyorthogonal to front and back surfaces 210 a, 210 b. A VCSEL is formed onfront surface 210 a and has an outer surface 218 a. The VCSEL includesgain layer 216 located between partially reflecting intermediate Braggreflector or mirror 214 and output Bragg mirror 218. The oxide layer 222that defines an aperture 222 a may be between the intermediate mirror214 and output Bragg mirror 218 on either side of the gain layer 216, ormay be on top of the output mirror 218. The head carrier is similar tothe prior art extended cavity VCSEL of FIG. 1 except that the thirdmirror, i.e., reflector or mirror 228 on back surface 210 b, is highlyreflective and output mirror 218 is less reflective than mirror 218(FIG. 1), which results in the laser radiation being output throughoutput mirror 218. Also, an annular isolation trench 223 separatesmirror 218 from region 210 c, which may be formed of the same materialas substrate 210. The isolation trench 223 forces current to flowthrough the aperture 222 a. An electrode layer 224 a is patterned onouter surface 218 a to provide electrical contact with region 210 c. Anelectrode layer 224 b is patterned on mirror 218 to provide electricalcontact with mirror 218. The electrodes 224 a, 224 b allow electricalcurrent to flow through the aperture 222 a. The semiconductor substrate210 with thickness L1 functions as an extended cavity for the VCSEL. Thesemiconductor substrate may be formed of GaAs or AlGaAs, with L1 in therange of about 500 to 1500 μm. The high reflectivity third mirror orreflector 228 may be formed of metal or a distributed Bragg reflector(DBR) may be used. The mirror 228 may have a reflectivity of about 99%to 99.9% and may cover either a limited area or the entire area of theslider leading edge. The intermediate Bragg mirror 214 may have areflectivity of about 50 to 90%. The output Bragg mirror 218 may have areflectivity of about 50 to 90% and has an outer surface 218 a which isthe outer surface of the VCSEL.

The TAR head is similar to the TAR head in the prior art of FIG. 2 butis fabricated on outer surface 218 a of the VCSEL over the electrodes224 a, 224 b, rather than on the trailing surface 112 of conventionalslider 110 (FIG. 2). The TAR head includes insulating layers 113,typically alumina, that generally surround the TAR head elements andprovide a portion of the ABS. An optional seed layer or protective layermay be deposited on the outer surface 218 a prior to the fabrication ofthe TAR head. During fabrication of the TAR head, vias (not shown) areformed in the insulating layers 113 to bring electrical connection ofelectrodes 224 a, 224 b out to connection pads 114 a, 114 b,respectively on outer surface 115, in the same manner that electricalconnection is made to the read head 60 and coil 56 of write head 50.

As shown in FIG. 3 the TAR head has an ABS that is substantiallycoplanar with the ABS of semiconductor substrate 210. An additionaldifference from the TAR head of FIG. 2 is grating coupler 77 that iscoupled to waveguide 72 and is oriented generally parallel to outersurface 218 a and orthogonal to radiation beam 230. The laser radiationbeam 230 passes from output mirror 218 through the insulating material113, which is transparent to the laser radiation at the wavelength,typically 920 to 1000 nm, to grating coupler 77. The grating coupler 77turns the incoming laser radiation 90 degrees and directs it intowaveguide 72. FIG. 4 is a view of grating coupler 77 and the taperedinput end 72 a of waveguide 72 as viewed from a direction orthogonal toouter surface 218 a. The grating coupler 77 is coupled to the taperedend 72 a of waveguide 72. Waveguide 72 is located between claddinglayers 73. Grating couplers are well-known and have been proposed foruse in TAR heads, as described for example in US 20090310459 A1.Focusing grating couplers and grating couplers coupled to taperedwaveguides are described by Van Laere, et al., “Compact Focusing GratingCouplers for Silicon-on-Insulator Integrated Circuits”, IEEE PHOTONICSTECHNOLOGY LETTERS, VOL. 19, NO. 23, DECEMBER1, 2007, pp. 1919-1921.

The front surface 210 a is the surface of a semiconductor wafer ontowhich a large number of VCSE1s and TAR heads are patterned. The wafer isthen diced into individual TAR head structures. FIG. 5 is a perspectiveview of a portion of a semiconductor wafer 270. The wafer 270 has agenerally planar front surface 210 a and back surface 210 b. Thematerials for forming the VCSELs are deposited and patterned on thefront surface 210 a, and the materials for forming the TAR heads aredeposited on the VCSELs. The material for third mirror 228 is depositedon the back surface 210 b. The VCSELs and TAR heads are patterned on aplurality of generally rectangular regions 280 that are arranged ingenerally parallel rows 290, with each region 280 being shown bounded bydashed lines 291, 292. Each region 280 is shown with an outline 293 thatrepresents the write head, which is typically the last portion of theTAR head fabricated on the wafer. After all the processing steps forforming the VCSELs and TAR heads, the wafer 270 is cut into rows 290along planes represented by dashed lines 291, and the rows 290 then cutalong planes represented by dashed lines 292, to form the individual TARhead structures. The structures are lapped, either at the row level orthe individual slider level, along planes parallel to planes representedby dashed lines 291, to define the ABS.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A thermally-assisted recording (TAR) head structure for a magneticrecording disk drive comprising: a semiconductor substrate having a backsurface and a front surface generally parallel to said back surface; avertical cavity surface emitting laser (VCSEL) on said front surface,the VCSEL having a laser radiation output surface generally parallel tosaid front surface, the laser radiation being output in a directionsubstantially perpendicular to said output surface; a back mirror onsaid back surface, the semiconductor substrate between the back mirrorand the VCSEL forming an extended cavity for the VCSEL; a TAR headformed on and in contact with the VCSEL output surface, the TAR headcomprising: a layer of electrically insulating material, the insulatinglayer having a disk-facing surface generally orthogonal to said back andfront semiconductor substrate surfaces; an optical waveguide orientedgenerally parallel to said output surface, the waveguide having agrating coupler for directing laser radiation from the output surface tothe waveguide; a near-field transducer (NFT) coupled to the waveguideand located at the disk-facing surface for receipt of laser radiationfrom the waveguide; a write head having a write pole at the disk-facingsurface; and a magnetoresistive read head having a sensing edge at thedisk-facing surface.
 2. The head structure of claim 1 wherein thesemiconductor substrate has a disk-facing surface substantially coplanarwith the disk-facing surface of said insulating layer.
 3. The headstructure of claim 1 wherein the semiconductor substrate comprises amaterial selected from GaAs and AlGaAs.
 4. The head structure of claim 1wherein the VCSEL comprises a partially-reflecting output mirror, apartially-reflecting intermediate mirror between said back mirror andthe output mirror, and an oxide layer having an aperture therein andlocated between the intermediate mirror and the output mirror.
 5. Thehead structure of claim 4 further comprising a first electrode forproviding electrical connection to the substrate and a second electrodefor providing electrical connection to the output mirror.
 6. The headstructure of claim 1 wherein the TAR head further comprises two shieldlayers of magnetically permeable material and wherein the read head islocated between the two shield layers.
 7. The head structure of claim 1wherein the write head further comprises a return pole magneticallycoupled to the write pole.
 8. The head structure of claim 1 wherein thewrite head further comprises an electrically conductive coil forgenerating magnetic flux in the write pole when electrical current ispassed through the coil.
 9. The head structure of claim 1 wherein thecoil is a helical coil wrapped around the write pole.
 10. The headstructure of claim 1 wherein the coil is a flat coil.
 11. The headstructure of claim 1 wherein the substantially coplanar disk-facingsurfaces of the semiconductor substrate and insulating layer are anair-bearing surface.
 12. A thermally-assisted perpendicular magneticrecording disk drive comprising: a perpendicular magnetic recording diskcomprising a disk substrate and a perpendicular magnetic recording layeron the disk substrate; and the head structure of claim 1; wherein thehead structure is maintained near the disk with the spacing between theNFT and the recording layer being less than the wavelength of the laserradiation.
 13. The disk drive of claim 12 wherein the perpendicularmagnetic recording layer is patterned into discrete data islands.